FM Transmitter

[Bob@FM]

I just called him, just a pair of Winegard TV antenna amps in series, he worked in a TV repair shop. Remember them?

If he worked in a service shop, maybe he hooked the TV amplifiers to the output of a stereo test generator.

BTW, that's also a good way to transmit FM stereo, provided the stereo generator has pre-emphasis.
Most of the stereo test generators are WAY better than anything based on couple chips inside, including the Ramsey. By a wide, wide margin. And by every measure - flatness, distortion, separation, dynamic range, etc.
Bob

 

[I LIKE MUSIC]

I agree that a quality FM stereo generator will produce clean signal. And as mentioned, one needs to be sure that the correct pre-emphasis is available.

The Ramsey transmitter transmitter and the C. Crane transmitter have reasonably okay limiters and compressors.

While it is not lab grade the Silicone Labs IC provides a good bang for the buck in terms of performance, it is better than most of the other jelly been FM transmitter ICs. I have built several FM broadcast band transmitters using the Silicone labs ICs. The last one was a step above the one used in the C, Crane transmitter. It includes the option to transmit RDS information and to directly feed it digital audio.

In general the ones that I built measured better than the published specs in most categories.

Again, it is what it is, but for a $40 FM transmitter, it is not all that bad. And the C. Crane transmitters that used the same Silicon Labs IC had good measured numbers.

BTW, I used a quality RF signal generator and drove it from my sound card via software ( a fairly nice 24/192 sound card). The software generates the multiplex audio signal and offers multiband audio processing, a range of limiters, compressors, EQ and so on.

I could throw the software into high gear and make myself sound like the big time one air talent or really bad or I could take part in the FM broadcast loudness wars..

One of the drawbacks of using an FM stereo generator is the lack of compression or limiting. Changing source material quite often means readjusting the audio level going into the RF signal generator. This can happen with in given play lists/source material.

 

[spicer]

Well, it is entirely possible that my experience with it was largely human error, too. Give it a go and see what you find. I think either the CCrane or the Ramsey would likely be better options, though. I'd believe anything PunkerX says about RF stuff, which biases me toward the 100b if I decide I want to play with a transmitter.

I am with you on PunkerX. What I knew about RF and antennas I have largely forgotten, just retain the ability to remember knowing those kinds of things... Anyway, I think I'll continue to read the informative posts before I buy anything. One thing that is appealing... very appealing... is the variable output measured at the high end in watts instead of MW. Back in the days of drive in theaters, when FM became almost standard in cars, and good aftermarket units became popular.. why theaters didn't use quality FM instead of Am for sound distribution... maybe it was power limitations, or dull management. I would have gone just for that...

 

[I LIKE MUSIC]

Higher power can be okay if you make sure to get the RF out of the area of the rest of your audio components, including whatever you are using as a source.

The unit that musichal used supplied a magnetic antenna mount he called a "stand".

For basic proper operation, it should have been placed on a metal surface at least 60 inches or so in diameter. Maybe he did and still had issues. but he did not mention it.


It likely did not supply an adequate ground plane for the needed feed line RF decoupling. But without an adequate ground plane to provide common mode RF decoupling of the antenna feed line, it is likely that common mode RF made it back to the audio source player. This is not necessary a good thing. If you read the reviews that I posted you will note the the addition and location of an external audio input cable made a large difference (much larger) in the measured RF field strength.

This is because of a large amount (not necessarily a good thing) of common mode RF current was on the shield of the audio cable. This means that the audio cable is acting as antenna and a conduit for RF current to get into the audio source device.

Even a 1 watt transmitter will produce around 2.3 million microvolts of RF field strength at 3 meters from the transmitter.

That is 2.3 volts of RF. That is a strong RF field strength to have in the vicinity of other audio gear.

At 1 watt the the RF output voltage of the transmitter into a 50 Ohm antenna will be in the ball park of about 7 volts.

Again that is a lot of RF voltage that may cause issues if not correct implemented.

If the antenna is not properly decoupled from the feed line, this RF becomes unwanted common mode RF current using the audio cable as a conduit to conduct it to the audio source (again, not necessarily a good thing).

I am not saying that everyone will have issues, because each installation is different.

This is just RF 101 and a heads up.

It is not unlikely that musichal might have had a more positive experience if his transmitter had been connected to an well tuned, well matched, and well feed line decoupled outside antenna, using well shield feed line, with the antenna located a fair distance from his audio gear and his audio source for the transmitter. Quality RF grounding (this is not the same as audio or power grounding) is not a bad idea.

Again this is just basic RF 101.

 

[62caddy]

I'm DirectTV subscriber and they have a number of themed music stations I enjoy, most of which are of very good audio quality. I've been considering ways of getting the audio signal to my other system in the sunroom which is used more often in warmer weather seasons.

Naturally the first thought would be run a good 50' ICs from the preamp REC OUT from my main system to the AUX input of the tuner/preamplifier of the other system. But this would also mean a lot of drilling into floors & joists and part of the IC would have to run outside (under the sunroom is a car port). I wonder how well one of these FM transmitter options would work... I assume it transmits in FM stereo?

According to the manufacturer, it is impossible to overload this model tuner's front end.

 

[I LIKE MUSIC]

62caddy, each situation can be different. Remember that it may not just be an issue with tuner RF overload. The things that I mentioned in my previous posts can raise their ugly heads and cause issues, especially with the higher power transmitters.

In general the lower the transmitter power, all other things being equal (and they may not be) the fewer RF issues as long as the RF field strength enough to make the trip to where you want it.

Yes, these transmitters do transmit in stereo.

 

[spicer]

I'm DirectTV subscriber and they have a number of themed music stations I enjoy, most of which are of very good audio quality. I've been considering ways of getting the audio signal to my other system in the sunroom which is used more often in warmer weather seasons.

Naturally the first thought would be run a good 50' ICs from the preamp REC OUT from my main system to the AUX input of the tuner/preamplifier of the other system. But this would also mean a lot of drilling into floors & joists and part of the IC would have to run outside (under the sunroom is a car port). I wonder how well one of these FM transmitter options would work... I assume it transmits in FM stereo?

According to the manufacturer, it is impossible to overload this model tuner's front end.

For something like that I would use this... https://www.monoprice.com/product?p_id=10247 ... from Monoprice. Simply don't use the video. For less than $50 on sale it is works great. Don't know how many walls you to go through between units but I have used a few of these to get audio from sources to a mixer and amp in a number of bar installations and they work perfectly. The center part is a lift up directional antenna for the 5.8 Ghz signal, direct them at each other, not real critical except maybe under difficult conditions... the audio is full spectrum, clean and quiet.

 

[62caddy]

For something like that I would use this... https://www.monoprice.com/product?p_id=10247 ... from Monoprice. Simply don't use the video. For less than $50 on sale it is works great. Don't know how many walls you to go through between units but I have used a few of these to get audio from sources to a mixer and amp in a number of bar installations and they work perfectly. The center part is a lift up directional antenna for the 5.8 Ghz signal, direct them at each other, not real critical except maybe under difficult conditions... the audio is full spectrum, clean and quiet.

Thanks for the suggestion. Do you think two double-sided plaster walls would pose an issue with transmission? Direct distance would be around 20-25' is my guess.

 

[I LIKE MUSIC]

Will one transmitter work with several of the matching receivers if one would like to send the audio to several locations?

If so,are separate receivers available?

I took look at the FAQs, apparently not. It appears to be just one to one.

 

[spicer]

Thanks for the suggestion. Do you think two double-sided plaster walls would pose an issue with transmission? Direct distance would be around 20-25' is my guess.

I don't know... double sided plaster is much more than two sheet rock boards. That's not a long distance though and I've been happy every time I've used one. One time the transmitter was buried inside a metal walled juke box and it had no problem with that and not being direct line of sight with the receiver at thirty feet apart.

 

[spicer]

Will one transmitter work with several of the matching receivers if one would like to send the audio to several locations?

If so,are separate receivers available?

I took look at the FAQs, apparently not. It appears to be just one to one.

I do believe it is one transmitter to one receiver. They do not offer multiple receivers.

 

[I LIKE MUSIC]

Thanks, I just wanted clarity for those that are looking for a multiple location solution.

 

[62caddy]

Sounds like it's worth trying. Certainly a lot simpler solution than a bulky FM transmitter and antenna. If it does video signal as well - that's even better.

 

[musichal]

Higher power can be okay if you make sure to get the RF out of the area of the rest of your audio components, including whatever you are using as a source.

The unit that musichal used supplied a magnetic antenna mount he called a "stand".

For basic proper operation, it should have been placed on a metal surface at least 60 inches or so in diameter. Maybe he did and still had issues. but he did not mention it.


It likely did not supply an adequate ground plane for the needed feed line RF decoupling. But without an adequate ground plane to provide common mode RF decoupling of the antenna feed line, it is likely that common mode RF made it back to the audio source player. This is not necessary a good thing. If you read the reviews that I posted you will note the the addition and location of an external audio input cable made a large difference (much larger) in the measured RF field strength.

This is because of a large amount (not necessarily a good thing) of common mode RF current was on the shield of the audio cable. This means that the audio cable is acting as antenna and a conduit for RF current to get into the audio source device.

Even a 1 watt transmitter will produce around 2.3 million microvolts of RF field strength at 3 meters from the transmitter.

That is 2.3 volts of RF. That is a strong RF field strength to have in the vicinity of other audio gear.

At 1 watt the the RF output voltage of the transmitter into a 50 Ohm antenna will be in the ball park of about 7 volts.

Again that is a lot of RF voltage that may cause issues if not correct implemented.

If the antenna is not properly decoupled from the feed line, this RF becomes unwanted common mode RF current using the audio cable as a conduit to conduct it to the audio source (again, not necessarily a good thing).

I am not saying that everyone will have issues, because each installation is different.

This is just RF 101 and a heads up.

It is not unlikely that musichal might have had a more positive experience if his transmitter had been connected to an well tuned, well matched, and well feed line decoupled outside antenna, using well shield feed line, with the antenna located a fair distance from his audio gear and his audio source for the transmitter. Quality RF grounding (this is not the same as audio or power grounding) is not a bad idea.

Again this is just basic RF 101.

I started to mention the mount was magnetic... but just writing "stand" fit the tone of my set-up guide better. I had read of one Amazon buyer who bought a twelve inch square of sheet metal for a ground plane. Based on that, I used a 16" pizza pan, having no idea that much larger was required, Should have suspected it maybe, since it looks like a car mount. Like I said in a previous post, there is a likelihood of user error. I read a lot before I made the purchase, and didn't find much practical, or knowledgeable info, it seems. Should have started a thread here before the purchase.

At any rate, UPS stopped by and picked it up late yesterday afternoon. This AM got a refund-issued notice from Amazon, so I had a learning experience that didn't cost me any money, with most of the learning here, after the fact. I feel better prepared if I buy another, and know to just post here if I need troubleshooting advice. Thanks to all who've responded, especially to I LIKE MUSIC for taking some RF-101 time. I very obviously need it.

And by all means, add to the discussion any other comments; it's interesting, informative and there are others out there as green and dumb as me that may benefit, but are afraid of looking stoopid, whereas I'm accustomed to that experience. So smoke 'em if you got 'em.

 

[I LIKE MUSIC]

The ground plane should be at least a quarter of a wavelength (about 30 inches for the FM broadcast band) in at least 4 directions from the antenna for best performance.

This is why the RF field strength increased for the units in the review when an audio input cord was connected and allowed to hang down. The audio cable became somewhat part of a vertical dipole, with the transmitter antenna attempting to be the other part of the dipole antenna.



As seen in the last picture, bending the radials down at about 45 degrees raises the feed point impedance from about 37 Ohms to about 50 Ohm to provide a little better antenna matching and it may help to reduce the unwanted RF common mode current.

It may not be such a big deal with the lower power FM transmitters, but with RF output power approaching 2 watts and for those that might run higher power transmitters at full power it can be an issue.

musichal, I have said many times that we all start out with exactly the same amount of knowledge, that is exactly zero, although I seem to have started in the negative region at less than zero.

For me, this information just seems to leak into my brain and get stuck, in place of normal useful information.

So smoke 'em if you got 'em.

Now you are making me feel old. I remember when that was a very common thing to say.

 

[musichal]

The ground plane should be at least a quarter of a wavelength (about 30 inches for the FM broadcast band) in at least 4 directions from the antenna for best performance.

This is why the RF field strength increased for the units in the review when an audio input cord was connected and allowed to hang down. The audio cable became somewhat part of a vertical dipole, with the transmitter antenna attempting to be the other part of the dipole antenna.

View attachment 1221352

As seen in the last picture, bending the radials down at about 45 degrees raises the feed point impedance from about 37 Ohms to about 50 Ohm to provide a little better antenna matching and it may help to reduce the unwanted RF common mode current.

It may not be such a big deal with the lower power FM transmitters, but with RF output power approaching 2 watts and for those that might run higher power transmitters at full power it can be an issue.

musichal, I have said many times that we all start out with exactly the same amount of knowledge, that is exactly zero, although I seem to have started in the negative region at less than zero.

For me, this information just seems to leak into my brain and get stuck, in place of normal useful information.



Now you are making me feel old. I remember when that was a very common thing to say.

My experience differs from yours. Opposite, in fact. I knew everything at birth, and still did at the age of 18. Then I began forgetting everything at an increasingly faster pace over the following decades.

 

[Bob@FM]

Here's another thing to consider if you are looking for an FM transmitter, especially if you are in the "more power is better" mode of thinking. That's great for audio power amps, but not so great for home based FM transmitters, especially if you are health conscious.

The FCC sets limit for human exposure to RF levels, which are summarized in this lengthy document here -
https://transition.fcc.gov/Bureaus/Engineering_Technology/Documents/bulletins/oet65/oet65.pdf

Just the the fact that the FCC sets limits on RF exposure should make you sit up and pay attention if you are monkeying around with a high power home based FM transmitter.

If you want the absolute levels, ship to appendix A at the end. But the one thing to remember is in the opening, and I quote -

The new FCC exposure limits are also based on data showing that the human body
absorbs RF energy at some frequencies more efficiently than at others. As indicated by Table 1
in Appendix A, the most restrictive limits occur in the frequency range of 30-300 MHz where
whole-body absorption of RF energy by human beings is most efficient. At other frequencies
whole-body absorption is less efficient, and, consequently, the MPE limits are less restrictive.

Bottom line, you may not want to put that 15W FM transmitter and antenna in your family room, or bedroom, or anywhere within 50-100ft or more of your (or anyone's) home. Use the lowest power possible to get the job done.

 

[musichal]

Thank god I always wear my tinfoil hat.

 

[Bob@FM]

Thank god I always wear my tinfoil hat.

I wonder if that would have helped workers in the Cuban Embassy?

 

[Bob@FM]

Here's another company to consider for an FM transmitter - I never tried any of their products, but they certainly appear to be well designed.
https://www.edmdesign.com/
Not US based though, pretty sure they are still in S. Africa

I use a product myself from Landmark Audio, the FM 350 transmitter.
https://fccid.io/STK-FM350

I see there's one for sale cheap with all the accessories here - this is a great deal, these were like $300-400 new
https://www.reach-prevalent.top/lan...us-fm-transmitter-w-power-supply-p-16656.html

Bob